An ordinary method for attaching a member to be fastened to a metal plate involves forming a bolt insertion hole in these two members, respectively, and then inserting a bolt into the holes to fix it by a nut. In the technical fields of automobiles and the like, however, there are also employed a weld bolt whose head part is welded and fixed onto a metal plate and a caulking bolt whose shaft part is inserted through a pilot hole formed in a metal plate and driven into the metal plate to caulk and fix the bolt head part to the metal plate, for simplifying the structure and decreasing the number of assembling steps.
For example, conventional caulking bolts are generally those including a head part 1 and a shaft part 2 wherein a rotation preventive protrusion 4 is formed on a seat surface 3 of the head part and a stopper ring 6 having a diameter larger than that of the shaft part 2 is formed between a normal complete thread part 5 of the shaft part 2 and the rotation preventive protrusion 4, as shown in FIGS. 1 and 2.
When such a caulking bolt is driven into a metal plate 8 in which a pilot hole 7 is formed as shown in FIG. 3, the metal is plastically deformed and flows between the seat surface 3 of the head part and the stopper ring 6 as shown in FIG. 4, so that the caulking bolt would not fall out in an axial direction. At the same time, the metal also flows into a concavo-convex part of the rotation preventive protrusion 4, so that the bolt cannot be rotated. Then, the member to be fastened can be attached to the caulking bolt thus fixed to the metal plate 8 by a nut.
However, since the normal complete thread part 5 is formed on a tip end side relative to the stopper ring 6, the distance between the seat surface 3 of the head part and the upper end of the normal complete thread part 5 is about 3.5 mm, for example, in the case of M6 bolt. Thus, when both of the metal plate 8 and the member 9 to be fastened are thin, fastening by a general nut 10, as shown in FIG. 5, cannot be performed due to an incomplete thread part formed at the upper end of the normal complete thread part 5. This problem can be solved by interposing a washer having a large plate thickness and/or using a nut having a special shape, but these solutions cause an increase in cost. Accordingly, conventional caulking bolts are disadvantageously hard to use in fixation of thin plates to each other.
Patent Literature 1 discloses a caulking bolt in which a concavo-convex part having both rotation preventing function and stopper function is formed between a seat surface of a head part and the upper end of a normal complete thread part. Also in this caulking bolt, however, the normal complete thread part is formed on the lower side of this concavo-convex part, and thus the distance between the seat surface of the head part and the upper end of the normal complete thread part is increased, so that the caulking bolt is disadvantageously hard to use in fixation of thin plates to each other, as with the above-described conventional caulking bolts.